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Full text of "Dual-Band VHF-UHF Twinlead J-Pole Antenna"

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J-Pole Antenna 

By James A. Williams 

W ant to get more range out of that 
handheld transceiver that just 
can't quite make that distant re- 
peater all your friends brag about? Or how 
about an emergency antenna that you can 
easily put up and use when a tornado or other 
natural disaster takes down your main an- 
tenna, or lightning causes you to have to 
disconnect that beam you have 60 feel in the 
air? Or do you live in an apartment or deed 
restricted subdivision that will not let you 
have outside antennas? 

If you have 30 minutes to 
spare, you can build a Dual- Band 
VHF/UHF J-Pole antenna that 
is guaranteed to gel you on the 
air quickly, outperform the rub- 
ber duck antenna that came with your handheld, 
and solve all the above problems and then 

It will serve not only as an emergency 
antenna, but as a portable antenna system for 
anyone needing an easy-to-sel-up antenna, 
such as travelers who want to monitor or 
operate from their hotel room. The low profile 
antenna can be set up and taken down quickly, 
and it stores in the space of an eyeglass case. 
It can be hung in a corner of a room, attic or 
other out of the way location for a more 
permanent installation, or can be put up and 
taken down when needed. 

The J-Pole antenna has been used by ham 
radio operators around the world for years. 
It’s one of the first antennas that most new 
hams hear about and wish to acquire. Several 
commercial antenna manufacturers make and 
sell this type of antenna in the price range of 
from $ 1 3 to S25 dollars each; you can make it 
for $3 or $4 dollars. 

54 inches 

Cut out small notch, 
remove only insulation 
Solder coax center conductor 

Loop of nylon line 
(to hang antenna) 

Standard Flat 
300 ohm Twinlead 

Cut out wire 
and insulation 
1/4" notch 

15.25" inches 

Cut out small notch, 
remove only insulation 
Solder coax braid (shield) 

1 .25" inches 

Twist wires together and solder 

For this article the we will concentrate on 
the most popular bands in this country: the 2 
meter band ( 144 MHz to 148 MHz) and the 70 
cm band(44()MH/to430MHz). I have found 
the antenna to have a voltage standing wave 
ratio (VSWR) of less than 1:3:1 across the 
entire 2-meter band, and less than 1:7:1 across 
the 70-cm band. As a scanner antenna (non- 
transmit) it is usable from 30 MHz through the 
gigahertz bands. 

I will not get into the theory or math in- 
volved in the construction of this antenna, 
other than to state that it is a vertically polar- 
ized antenna with two elements: a 3/4 wave- 

length radiator and a 1/4 wave length match- 
ing stub that operates like an end-led halfwave 
antenna. If built to exact specs listed, there is 
no tuning needed for this antenna. Just build it 
and use it. 

The Dual-Band Twinlead J-Pole antenna 
is made from standard flat 300 ohm twinlead 
TV antenna wire, available from most hard- 
ware and electronic stores. Most of us oldtimers 
will remember this wire as the twinlead used 
to connect a TV to the roof-mounted antenna 
that almost all homes in this country dis- 
played before the advent of cable TV. You can 
use an old piece of twinlead you or a friend 


may have lying around or buy a new piece 6 
fool long. 

You will need the following: 

6 feet of flat 300 ohm twinlead 

6 feet or more of RG-58 coax 

PL-259 or BNC connector (your choice) 

Soldering Iron and Solder 

Wire Cutters, strippers or knife 

Plastic electrical tape or heat shrink tubing 

■ Building the Antenna 

Begin by carefully removing 0.75 inch of 
insulation from one end of the twinlead. being 
careful that you also do not cut the wires. Once 
the wire has been stripped, twist the pair of 
wires together and solder. This is the bottom 
of the antenna. 

Cut the twinlead to 54 inches exactly from 
top to bottom. Double check the length: it 
needs to be exact. 

Measure 1 .25 inches up from the bottom of 
the antenna and mark the insulation. From this 
mark, carefully remove the insulation for .25 

inches above and below the mark, being care- 
ful not to nick the wire. Repeat on the other 
side of the twinlead. 

Measure exactly 1 5.25 inches from the one 
of the notches you have just cut and remove a 
.25 inch notch from this point; remove both 
the insulation and the wire. This is the ground 
side (1/4 wave stub). The other uncut side is 
the radiator side (3/4 wave) of the antenna. 

Using a piece of RG-58 coax at least 6 feet 
long, carefully remove .75 inches of insula- 
tion from the coax to expose the braid. Sepa- 
rate the braid from the center conductor and 
twist together. Remove the insulation from 
the center conductor. Connect the braid from 
the coax to the ground side (stub) of the 
twinlead and the center conductor to the long 
side (radiator) of the twinlead. Try to connect 
the coax so that it lays in the center of the 
twinlead. Solder the coax braid and center 
conductor to the twinlead. 

Use electrical tape or heat shrink tubing to 
secure the coax to the twinlead. This will help 
lake the stress offthe solder points and make 
the joint waterproof. Also tape or use heat 

shrink tubing at the notch you cut out to make 
the ground side (stub) of the antenna. 

Punch a hole at the top of the antenna at the 
center and tie a loop of nylon line (fishing) or 
another nonconducting line to hang the an- 
tenna from the ceiling, tree limb or other 
location. Install your choice of connector (PL- 
259. BNC. etc.) to the coax and use. You will 
find that the antenna will have a gain of 3 to 4 

When using the antenna try to keep it away 
from metallic objects such aselectrical wiring 
and plumbing. It is possible to detune the 
antenna if it is close to such objects. For a 
permanent outdoor installation, mount the J- 
Pole inside a piece of PVC pipe capped at both 
ends. Drill a hole in the bottom cap for the 
coax to pass through and seal around the coax 
with a good waterproof sealer. Mount on top 
of your mast or tower. 

By building several of these low cost, easy 
to construct antennas you can keep one handy 
at all times — such as in your car glove com- 
partment. briefcase or back pocket — and en- 
joy the improved performance of this antenna 

Solder coax 
center conductor” 
to wire 

Tape or Heat Shrink Tubing 
Coax to Twinlead. Try to 
center coax on twinlead 
as shown. 

Standard Flat 
300 ohn Twinlead 

Solder coax braid 
(shield) to wire 


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